(1) Field of the Invention
The present invention generally relates to an oxygen sensor of an internal combustion engine, and more particularly to an apparatus for controlling the heating of a current-limiting type oxygen sensor which is adapted to determine the oxygen content in combustion exhaust gases and especially in the exhaust gases from internal combustion engines of automotive vehicles.
(2) Description of the Related Art
Generally, outputs of an oxygen sensor which determine the oxygen concentration of exhaust gases from an internal combustion engine vary depending on the temperature of a sensing portion to which the oxygen sensor is exposed. It has been proposed to heat the oxygen sensor and to control the heating of the oxygen sensor so that it will be operating at an essentially constant temperature.
U.S. Pat. No.4,419,190 (corresponding to Japanese Published Patent Application No.4-24657) discloses a proposed apparatus to measure an operating temperature of a current-limiting type oxygen sensor. In the proposed apparatus, an ac signal from an ac voltage source is superimposed on a dc signal from a dc voltage source, and it is applied to two electrodes of a current-limiting type sensor, and the magnitude of output current flowing through the sensor is measured so that the operating temperature of the sensor which is highly dependent on the alternating current resistance of the sensor is determined. The proposed apparatus controls the heating of the sensor in accordance with the determined temperature of the sensor, so that it will be operating at an essentially constant temperature. The proposed apparatus will thus realize a device for controlling the heating of the current-limiting type oxygen sensor with no need for an additional temperature sensor, the temperature sensor used to sense the operating temperature of the oxygen sensor.
However, it is necessary that the proposed apparatus mentioned above uses an ac voltage source which generates an ac signal to be superimposed on the dc signal, in order to determine the operating temperature of the sensor. Also, it is necessary that the proposed apparatus includes a comparator to compare the amplitude of the alternating voltage from the ac voltage source with the amplitude of the alternating voltage from the sensor with a high accuracy. Thus, the proposed apparatus has a complicated circuit structure with additional circuit components, and the cost of manufacture will be increased.